Currently Unraid isn't meant to be resold, and as such if you buy the license under your credentials, it remains your license. Reselling is against the TOS.
However... I get where you are wanting to go, and if you are up front with your clients that the hardware is theirs, but the license remains yours and they are ok with you being the only point of contact for official support, then I see no issue in doing what you propose. The "my servers" credentials aren't used to actually manage the servers, the root password is still needed. My servers is just a way to get to the management login remotely, and the clients should only need to manage the servers locally. If they need remote management as well, I suggest setting them up with the wireguard VPN that only connects directly to their server.
Disclaimer: I am NOT a Limetech employee, my statements are based on what I have learned and experienced over many years. If you wish to discuss resale of licenses you will need to talk to Limetech directly.
I would strongly suggest not doing what you are proposing, only because the ongoing support to keep your clients happy is likely to be more time consuming than they are willing to pay for. Don't get me wrong, I love Unraid and have helped a few select clients set up their own Unraid machines, but they bought the licenses and manage things themselves with minimal handholding. I do not promote Unraid as a typical business solution, only because somebody on the payroll needs to be running it, the ongoing support for an office with only basic tech experience would quickly sour the client on the support needs.
@jonp would be a good person to contact if you want a more official answer.